Gert Bachmann

Physiological and ecological aspects of the interactions between plant roots and rhizosphere soil

Abstract The rhizosphere soil of six different plant species grown in a factorial combination with four different types of soil was analysed to gain information about the amount of organic metabolites and their origin in the rhizosphere. Amino-acids, sugars, CO 2 evolution and some enzyme activities were measured quantitatively. The results were compared using various statistical methods such as multiple correlation, factor analysis, principal component analysis, and cluster analysis. The amounts of sugars and aminoacids, and the enzyme activities of some metabolites behave in an interdependent way, suggesting a very close interrelationship of the involved organisms. In most cases the soil was the dominating factor in the combination. Some plants were able to predominate certain soil factors. Emphasis is laid on the importance of such physiological investigations as a diagnostic tool for the assessment of the biological state of various natural agricultural soils.

Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil

In a field study using soil mesocosms in an acid spruce forest soil we investigated the effects of mesofauna and macrofauna on microbial biomass, dissolved organic matter, and N cycling. Intact soil monoliths were taken from the ground, defaunated by deep-freezing, and wrapped in nets of various mesh-sizes to control re-immigration of different faunal size-classes. The monoliths were then replanted in the field. Three treatments of mesocosms were prepared: (1) with only microbiota, (2) microbiota and mesofauna, and (3) microbiota, mesofauna, and macrofauna (= complex fauna). After 8 months of exposure the mesocosms and the unmanipulated control plots (treatment 4) were destructively sampled. We estimated microbial biomass by substrate-induced respiration and the chloroform fumigation-extraction method. N cycling was measured by monitoring microbial N mineralization, the NHinf4sup+content, and selected amino acids and the activities of protease, urease, and deaminase. The results from the L/F layer showed that the pool of the microbial biomass was not changed by the activity of the mesofauna. However, the mesofauna and macrofauna together enhanced SIR. An increase in microbial N mineralization was only observed in treatment 3 (microbiota + complex fauna). Protease activity and NHinf4sup+content increased in treatments 2 (microbiota + mesofauna) and 3 (microbiota + complex fauna). The complex fauna induced a soil pH increase in treatment 3 as opposed to treatment 1 and the control. This increase was presumably due to excretory NHinf4sup+. Principal component analysis revealed that the complex fauna in treatment 3 caused a significantly higher N turnover per unit of microbial biomass.

Plant species affect the concentration of free sugars and free amino acids in different types of soil

Substantial amounts of low molecular weight organic compounds (LMWs) such as sugars and amino acids are transferred from plant roots into soil. These substances are released due to decomposition processes or leaching (exudation). Afterwards they can be metabolized by soil microorganisms into different compounds, or they can be partially re-absorbed by the plants. The aim of this study was to clarify the influence of five wild plant species on the composition and pool sizes of LMWs extractable from three different soils. Four of the five species caused significant changes in soil LMW pools. In Chernozem, the sugar concentrations of soil with plants were up to 60 % higher than those of the bulk reference soil, and amino acids increased by as much as 207 %. The relative abundance of free amino acids in roots did not correlate with the relative abundance of amino acids in soil after six weeks of plant growth. The relative abundance of soil amino acids, that increased after plant growth, was strongly dependent on the type of soil and on the plant species present. We suggest that rather than rhizodeposition being dependent on soil type, it reflects differential microbial metabolization of amino acids in the respective soils. Pflanzen verandern die Konzentration freier Zucker und Aminosauren im Boden Betrachtliche Mengen von niedermolekularen organischen Substanzen (NMOS) (wie z.B. Zuckern und Aminosauren) werden von Pflanzenwurzeln in den Boden abgegeben. Dies geschieht durch Wurzel-Abbau und durch passives Austreten. Ein Teil dieser Substanzen wird von Mikroorganismen metabolisiert und wieder ausgeschieden. Es kann auch zu einer Wiederaufnahme durch Pflanzenwurzeln kommen. Das Ziel dieser Arbeit war, den Einfluss von funf Wildpflanzenarten auf die Zusammensetzung und Menge von extrahierbaren freien Aminosauren und Zuckern in drei Bodentypen zu messen. Vier der funf Arten fuhrten zu signifikanten Anderungen der NMOS. Im Tschernosem stieg die Gesamtkonzentration der im Boden frei vorliegenden Zucker nach Bepflanzung um bis zu 60 % an, jene der Aminosauren war um bis zu 207 % hoher als im unbepflanzten Boden. Der relative Anteil der einzelnen Aminosauren in den Wurzeln korrelierte nicht mit der relativen Haufigkeit jener freien Aminosauren im Boden, die nach sechs Wochen Bepflanzung signifikant anstiegen. Das Haufigkeitsmuster freier Aminosauren im Boden, die nach Bepflanzung zunahmen, war signifikant abhangig vom Bodentyp, aber auch von der jeweiligen Pflanzenart. Eine mogliche Deutung fur die variierenden relativen Aminosaure-Haufigkeiten ware, dass in den jeweiligen Boden eine unterschiedliche mikrobielle Metabolisierung dieser Aminosauren erfolgt.

Evidence for a rhizobia-induced drought stress response strategy in Medicago truncatula.

Drought stress hampers plant energy and biomass production; however it is still unknown how internal C:N balance and rhizobial symbiosis impact on plant response to water limitation. Here, the effect of differential optimal nitrogen nutrition and root nodule symbiosis on drought stress and rehydration responses of Medicago truncatula was assessed. Two groups of plants were nodulated with Sinorhizobium medicae or Sinorhizobium meliloti--differing in the performance of N fixation; the third group grew in a rhizobia-free medium and received mineral nitrogen fertilizer. In addition to growth analyses, physiological and molecular responses of the two systems were studied using ionomic, metabolomic and proteomic techniques. We found a significant delay in drought-induced leaf senescence in nodulated relative to non-nodulated plants, independent of rhizobial strain and uncoupled from initial leaf N content. The major mechanisms involved are increased concentrations of potassium and shifts in the carbon partitioning between starch and sugars under well-watered conditions, as well as the enhanced allocation of reserves to osmolytes during drought. Consequently, nodulated plants recovered more effectively from drought, relative to non-nodulated M. truncatula. Proteomic data suggest that phytohormone interactions and enhanced translational regulation play a role in increased leaf maintenance in nodulated plants during drought.

Versatile Redox Chemistry Complicates Antioxidant Capacity Assessment: Flavonoids as Milieu-Dependent Anti- and Pro-Oxidants

Some antioxidants have been shown to possess additional pro-oxidant effects. Diverse methodologies exist for studying redox properties of synthetic and natural chemicals. The latter are substantial components of our diet. Exploration of their contribution to life-extending or -compromising effects is mandatory. Among reactive oxygen species (ROS), hydroxyl radical (•OH) is the most damaging species. Due to its short half-life, the assay has to contain a specific generation system. Plants synthesize flavonoids, phenolic compounds recognized as counter-agents to coronary heart disease. Their antioxidant activities are affected by their hydroxylation patterns. Moreover, in the plant, they mainly occur as glycosides. We chose three derivatives, quercetin, luteolin, and rutin, in attempts to explore their redox chemistry in contrasting hydrogen peroxide environments. Initial addition of hydrogen peroxide in high concentration or gradual development constituted a main factor affecting their redox chemical properties, especially in case of quercetin. Our study exemplifies that a combination of a chemical assay (deoxyribose degradation) with an electrochemical method (square-wave voltammetry) provides insightful data. The ambiguity of the tested flavonoids to act either as anti- or pro-oxidant may complicate categorization, but probably contributed to their evolution as components of a successful metabolic system that benefits both producer and consumer.

Combining microdilution with MicroResp™: Microbial substrate utilization, antimicrobial susceptibility and respiration

Pharmacological studies focus on susceptibility of pathogenic microbes against specific drugs or combinations of them, ecological studies on substrate utilization efficiency of variable microbial communities. The MicroResp™ system was especially developed to study soil microbial communities. It was slightly modified to facilitate exploring of microbial growth efficiency in a concentration-dependent fashion (microdilutions of carbohydrate mixtures or specific toxic chemicals). After turbidimetric growth assessment, colorimetric indicator plates (cresol red agar) were mounted to the assay plates. The substrate utilisation design is illustrated by glucose and a plant carbohydrate mixture, the antimicrobial susceptibility design by the naphthoquinone juglone. Dose-response effects are explored by curve fitting of nonlinear models that especially have been developed to detect hormetic effects that are characterized by stimulation at lower followed by inhibition at higher dosages (U- and inverse U-shaped effects). Multivariate analyses are presented utilizing metavariables that were obtained in the curve fitting process of the measured parameters growth and respiration and the factor growth efficiency.

Comprehensive tissue-specific proteome analysis of drought stress responses in Pennisetum glaucum (L.) R. Br. (Pearl millet)☆

UNLABELLED Pearl millet is the fifth most important cereal crop worldwide and cultivated especially by small holder farmers in arid and semi-arid regions because of its drought and salt tolerance. The molecular mechanisms of drought stress tolerance in Pennisetum remain elusive. We have used a shotgun proteomics approach to investigate protein signatures from different tissues under drought and control conditions. Drought stressed plants showed significant changes in stomatal conductance and increased root growth compared to the control plants. Root, leaf and seed tissues were harvested and 2281 proteins were identified and quantified in total. Leaf tissue showed the largest number of significant changes (120), followed by roots (25) and seeds (10). Increased levels of root proteins involved in cell wall-, lipid-, secondary- and signaling metabolism and the concomitantly observed increased root length point to an impaired shoot-root communication under drought stress. The harvest index (HI) showed a significant reduction under drought stress. Proteins with a high correlation to the HI were identified using sparse partial least square (sPLS) analysis. Considering the importance of Pearl millet as a stress tolerant food crop, this study provides a first reference data set for future investigations of the underlying molecular mechanisms. BIOLOGICAL SIGNIFICANCE Drought stress is the most limiting factor for plant growth and crop production worldwide. At the same time drought susceptible cereal crops are among the largest producers worldwide. In contrast, Pearl millet is a drought and salt tolerant cereal crop especially used in arid and semi-arid regions by small farmers. The multifactorial molecular mechanisms of this unique drought tolerance are not known. Here, we employ shotgun proteomics for a first characterization of the Pearl millet drought stress proteome. The experimental setup and the data set generated from this study reveal comprehensive physiological and proteomic responses of the drought stressed Pearl millet plants. Our study reveals statistically significant tissue-specific protein signatures during the adaptation to drought conditions. Thus, the work provides a first reference study of the drought stress proteome and related drought responsive proteins (DRPs) in Pearl millet.

Pro- and Antioxidant Activity of Three Selected Flavan Type Flavonoids: Catechin, Eriodictyol and Taxifolin

The flavanol (±)-catechin shows an OH group but no 4-keto group on ring C (C3), and no conjugation between ring A and B. The related flavanone (+)-eriodictyol has a keto group on C4 but no 3-OH group on ring C. (+)-Taxifolin, another flavanone, has an OH on C3 and a keto group on C4 of the C ring. Deoxyribose degradation assay systems, with hydrogen peroxide and ascorbic acid either added or omitted, were performed in variants in which Fe(III) was added in a complex with ethylenediaminetetraacetic acid (EDTA). In combination with differential pulse voltammetry (DVP), the specific redox-chemical contributions of the ring A m-dihydroxyl groups could be explored more specifically in addition to those of the traditionally investigated o-dihydroxyl groups of ring B.

Low-molecular-weight metabolite systems chemistry

Low-molecular-weight metabolites (LMWMs) comprise primary or central and a plethora of intermediary or secondary metabolites, all of which are characterized by a molecular weight below 900 Dalton. The latter are especially prominent in sessile higher organisms, such as plants, corals, sponges and fungi, but are produced by all types of microbial organisms too. Common to all of these carbon molecules are oxygen, nitrogen and, to a lesser extent, sulfur, as heteroatoms. The latter can contribute as electron donators or acceptors to cellular redox chemistry and define the potential of the molecule to enter charge-transfer complexes. Furthermore, they allow LMWMs to serve as organic ligands in coordination complexes of various inorganic metals as central atoms. Especially the transition metals Fe, Cu and Mn can catalyze one electron reduction of molecular oxygen, which results in formation of free radical species and reactive follow-up reaction products. As antioxidants LMWMs can scavenge free radicals. Depending on the chemical environment, the same LMWMs can act as pro-oxidants by reducing molecular oxygen. The cellular regulation of redox homeostasis, a balance between oxidation and reduction, is still far from being understood. Charge-transfer and coordination complex formation with metals shapes LMWMs into gel-like matrices in the cytosol. The quasi-polymer structure is lost usually during the isolation procedure. In the gel state, LMWMs possess semiconductor properties. Also proteins and membranes are semiconductors. Together they can represent biotransistor components that can be part of a chemoelectrical signaling system that coordinates systems chemistry by initiating cell differentiation or tissue homeostasis, the activated and the resting cell state, when it is required. This concept is not new and dates back to Albert Szent-Gyorgyi.

Nutrient Turnover, Greenhouse Gas Exchange and Biodiversity in Natural Forests of Central Europe

We measured microbial turnover of carbon (C) and nitrogen (N) in 12 natural forest reserves in Austria, and estimated potential emission rates of nitrous oxide (N2O) and carbon dioxide (CO2), and uptake rates of methane (CH4). The community composition of soil microorganisms was investigated using PLFA (phospholipid fatty acid) analysis and molecular tools, and we examined the biodiversity of selected taxa of micro-, meso- and macrofauna. These characterizations of natural forests provide reference data for evaluating soil biology in managed, especially disturbed or damaged forests. Ecophysio-logical quotients were tested for their ability to make predictions about the carbon dynamics of forest soils. The 12 forests represented the six typical types in Central Europe: oak, beech, spruce-fir-beech, floodplain, and pine forests. Nitrogen turnover rates were high in moist soils with high pH. Nitrogen losses as nitrate or N2O were small unless N deposition exceeded 30 kg ha−1 yr−1. The fastest turnover of C and N occurred in the floodplain forests, based on microbial quotients, xylanase activity, the relative thickness of litter layer and 15N abundance in the organic soil. Carbon turnover was slowest in the beech forests on acidic bedrock, and slow turnover may lead to the largest net C accumulation. Tree species had distinct effects on microbial communities, but high soil biodiversity in these natural forests may not be greater than in managed forests.